1. Field of the invention
The present invention relates to a toner for developing electrostatic images, used in electrophotography, electrostatic printing or electromagnetic recording and suited for heat fixing. It also relates an image forming apparatus and a facsimile apparatus that make use of such a toner.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a recording medium such as paper if necessary, followed by fixing by the action of heat, pressure or solvent vapor.
Various methods or apparatus have been developed in relation to the above final step, i.e., the step of fixing the toner image to a sheet-like recording medium such as paper. A method most commonly available at present is the pressure heating system making use of a heating roller.
The pressure heating system making use of a heating roller is a method of carrying out fixing by causing a recording medium to pass over a heating roller whose surface is formed of a material having a releasability to toner while a toner image surface of the former is brought into contact with the surface of the latter under application of a pressure. Since in this method the surface of the heating roller comes into contact with the toner image of the recording medium under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the recording medium, so that fixing can be carried out rapidly. This method is therefore very effective in high-speed electrophotographic copying machines. In this method, however, since the surface of the heating roller comes into contact with the toner image under application of a pressure in the latter's molten state, part of the toner image may adhere and transfer to the surface of the fixing roller, which may re-transfer to the subsequent recording medium to cause an offset phenomenon, resulting in a contamination of the recording medium. Thus, it is essential in the heating roller fixing system that no toner is adhered to the surface of the heat fixing roller.
For the purpose of not causing the toner to adhere to the surface of a fixing roller, a measure has been hitherto taken such that the roller surface is formed of a material such as silicone rubber or a fluorine resin, having an excellent releasability to toner, and, in order to prevent offset and to prevent fatigue of the roller surface, its surface is further covered with a thin film formed using a fluid having a good releasability as exemplified by silicone oil. However, this method, though effective in view of the prevention of the offset of toner, requires a device for feeding an anti-offset fluid, and hence has the problem that the fixing device becomes complicated.
Hence, it is not preferable to prevent offset by feeding the anti-offset fluid and, under existing circumstances, it is sought to make an advancement on a toner having a broad fixing temperature range and high anti-offset properties. Now, a method has been taken in which a wax such as a low-molecular weight polyethylene or a low-molecular weight polypropylene capable of melting well upon heating is added in the toner so that the toner can have an improved releasability. Its addition can be effective for preventing offset, but on the other hand the wax tends to cause a lowering of durability because agglomerating properties of the toner may increase and charge performance thereof may become unstable. Various attempts have been made on improving binder resins.
For example, a method is known in which, in order to prevent offset, a binder resin for a toner is made to have a higher glass transition temperature (Tg) or molecular weight so that the toner can have an improved melt viscoelastisity. However, an attempt to improve anti-offset by such a method may bring about an unsatisfactory fixing performance to cause the problem that the fixing performance in a low-temperature environment, i.e., low-temperature fixing performance, that is required of high-speed copying machines and energy saving may become poor.
In order to improve the fixing performance of toners, it is necessary to decrease the viscosity of toner at the time of melting so that the toner has a large contact area with respect to a fixing substrate. For this reason, it is required for the binder resin used therein to have a low Tg and molecular weight.
That is, the low-temperature fixing performance and the anti-offset are contradictory in one aspect, and hence it is very difficult to give a toner that can satisfy these functions at the same time.
To solve this problem, Japanese Patent Publication No. 51-23354, for example, discloses a toner comprising a vinyl polymer cross-linked at an appropriate degree by adding a cross-linking agent and a molecular weight modifier. A number of proposals are also made on blend toners wherein Tg, molecular weight and gel content are controlled in combination in vinyl polymers.
Toners containing such a cross-linked vinyl polymer or a gel component can certainly improve anti-offset properties.
However, when they are incorporated into the toner, use of this cross-linked vinyl polymer as a material for toner provides very great internal friction in the polymer when materials are melt-kneaded during the preparation of toner, where a large shear force is applied to the polymer. Hence, in many instances, molecular chains are cut which cause a decrease in melt viscosity, so that the anti-offset properties are adversely affected.
Now, as measures to solve this problem, Japanese Patent Applications Laid-open No. 55-90509, No. 57-178249, No. 57-178250 and No. 60-4946 disclose that a resin containing a carboxylic acid and a metal compound are used as materials for a toner, which are heated and reacted when melt-kneaded, to form a cross-linked polymer incorporated into the toner.
Japanese Patent Applications Laid-open No. 63-214760, No. 63-217362 and No. 63-217363 also disclose that a vinyl monomer, a vinyl resin having a specific half ester compound as an essential component unit, and a polyvalent metal compound are allowed to react to effect cross-linking.
Japanese Patent Applications Laid-open No. 63-214760, No. 63-217362 and No. 63-217363 also disclose that molecular weight distribution of the resin is separated into two groups comprised of a low-molecular weight and a high-molecular weight fraction, where a carboxyl group of a specific half ester compound contained in the low-molecular weight side is reacted with a polyvalent metal ion.
In proposals in these disclosures, however, carboxylic acids and metal compounds are used as starting materials so as to form cross-links. Many of these materials have a strong, negative chargeability or if a weak, negative chargeability, they have a strong negative chargeability in a decomposition product formed as a result of the cross-linking reaction. Accordingly, although they are effective for negatively chargeable toners, they are not preferable in many respects such that a decrease in density and fogging occurs when they are used in positively chargeable toners.
Thus, effective incorporation of the cross-linked polymer and gel component into toners requires use of materials with negative chargeability and, under existing circumstances, it is sought to provide a cross-linking material suited for positively chargeable toners.
Moreover, the toners proposed in these disclosures have not satisfied the performances required of toners, in particular, the anti-offset properties required for high-speed machines. For example, in high-speed machines making copies of 80 or more sheets per minute, no satisfactory fixing performance can be achieved and there is another problem that recording mediums are stained by the toner which flows out of a cleaning member provided in contact with a fixing roller.
More specifically, in such high-speed machines making copies of 80 or more sheets per minute, the offset material on the fixing roller amounts to a considerable quantity because of an enormous number of paper feed, even though the quantity of offset per sheet is very small. This may cause troubles for fixing assemblies. In order to remove this small quantity of offset matters, a fixing-roller cleaning member such as a cleaning roller or web made of silicone rubber is fitted in contact with a fixing roller. Conventional binder resins for toners are mainly so designed as to accomplish low-temperature fixing performance and anti-offset, and not so designed as to maintain a high melt viscosity to a temperature as high as 200.degree. C. or above. Hence, the toner components having adhered to the fixing-roller cleaning member developed a low melt viscosity because of their residence thereon for a long time at the temperature set for the fixing roller. Moreover, in an instance in which the temperature of the fixing roller has overshot the temperature set for the fixing roller when, e.g., a copying machine is switched on, the fixing roller develops a temperature of 200.degree. C. or above to cause an extreme decrease in the melt viscosity of adhering toner components, which are again transferred to the fixing roller to stain recording mediums.